1. Field of the Invention
The present invention relates to an inhibitor of lead-induced stress corrosion cracking in the secondary side of steam generator tubes in nuclear power plants, comprising nickel boride, and an inhibition method using the same.
2. Description of the Related Art
Generally, nickel alloys are widely known to be susceptible to corrosion damage, such as intergranular attack (IGA) or intergranular stress corrosion cracking (IGSCC), under caustic environments.
Further, after lead was reported to affect the stress corrosion cracking resistance of Inconel (Alloy) 600 in the 1960s [H. R. Copson and S. W. Dean, Corrosion, 21(1), 1, 1965], lead-induced stress corrosion cracking was observed in the secondary side of steam generator tubes during operation of nuclear power plants since the 1980s. Furthermore, in the steam generator of a Kori-2 nuclear power plant, Korea, cracking which is presumed to have been primarily induced by lead was also observed. Such lead-induced stress corrosion cracking is characterized in that transgranular stress corrosion cracking (TGSCC) is observed along with intergranular stress corrosion cracking (IGSCC), unlike the kind of cracking mode that occurs due to sulfides or under caustic environments without lead. As such, the cause of introducing a lead component, which gives rise to lead-induced stress corrosion cracking, into the secondary side of the steam generator are known to be metallic streaks left by the use of lead mallets, plant makeup water, condenser leaks, copper alloys in the feed streams, grease & lubricants, seals & gaskets, babbitt alloys, preservatives & paints, marking pencils, lead chromate tinting in polyethylene wrapping materials, brazes & solder used on pipes, radiation shielding materials, etc. Typically, although the amount of lead supplied from the above materials is very small, lead may locally be concentrated at a high level ranging from hundreds to thousands of ppm or more in sludge deposited on tubesheets and in the niche between the tubes. In this way, the lead component, which is introduced into the secondary side of the steam generator and concentrated there, directly takes part in the steam generator tube, undesirably resulting in lead-induced stress corrosion cracking.
Inconel (Alloy) 600 may be susceptible to stress corrosion cracking in an acidic solution, a neutral solution and a caustic solution, and lead pollution functions to further accelerate stress corrosion cracking [(M. Helie, “Lead Assisted Stress Corrosion Cracking of Alloys 600, 690 and 800”, Proceeding of the 6th International symposium on environmental degradation of materials in nuclear power systems-water reactors, San Diego, Calif., August 1-5 179, 1993), (S. S. Hwang, H. P. Kim, D. H. Lee, U. C. Kim and J. S. Kim, “The mode of Stress Corrosion Cracking in Ni-base alloys in High Temperature Water Containing Lead”, Journal of Nuclear Materials, 275, 28, 1999)], and thus may result in fatal damage to steam generator tubes.
After the first report, in which not IGSCC but TGSCC occurs in Inconel 600 in hot water containing lead [H. R. Copson and S. W. Dean, Corrosion, 21(1), 1, 1965], it was reported that conditions able to induce the most stress corrosion cracking in the Inconel 600 due to lead are the temperature ranging from 300° C. to 350° C. and the pH about 10 [S. Suzuki, “IGA resistance of TT Alloy 600 and concentration behavior of broached egg crate support construction”, Proceedings of the 5th International Symposium on Environmental Degradation of Materials in Nuclear Power Systems-Water Reactors, Monterey, 861, 1991], and also that the most extreme cracking is caused by addition of lead in the form of PbO, among pure Pb powder, PbO, PbCl2, Pb3(PO4)2, and PbSO4.
One of the accidents occurring in the steam generator tubes due to the lead-induced stress corrosion cracking is the leakage of primary side cooling water contaminated with radioactive material to the secondary, which is considered to be a direct factor leading to the unscheduled stoppage of operation of nuclear power plants, repair of damaged steam generator tubes, or exchange of steam generators themselves, thereby causing considerable economic loss. Further, an impurity concentration in sludge piled on the top of tubesheet during operation and in the niches between the steam generator tubes, and thus highly caustic or neutral conditions are formed, resulting in stress corrosion cracking of the tubes. Furthermore, attributable to lead supplied through various routes, corrosion damage of the tubes may be further accelerated. Therefore, in order to inhibit or prevent stress corrosion cracking damage to the steam generator tubes during operation, a lot of research effort has been directed toward inhibitors or methods of inhibiting stress corrosion cracking in the secondary side. The conventional inhibitor of stress corrosion cracking is exemplified by TiO2, TiB2, CeB6, and LaB6. In the case of TiO2, it was applied onsite in practice to power plants.
Korean Patent No. 415265 discloses a method of inhibiting stress corrosion cracking in the secondary side of steam generator tubes in nuclear power plants, in which a compound selected from the group consisting of cerium boride, lanthanium boride and mixtures thereof is supplied to secondary cooling water. This method can increase the resistance to stress corrosion cracking of the tubes three times or more compared to other methods, and two times or more compared to conventional corrosion inhibitors, such as boric acid and titanium oxides. In addition, as disclosed in Korean Patent No. 609590, nickel boride can reduce the incidence of stress corrosion cracking of a test specimen simulating a steam generator tube in a nuclear power plant under a highly caustic conditions compared to a reference solution, and can decrease the corrosion current density and the thickness of oxide film to thus increase corrosion resistance. In the present invention, disclosed are an inhibitor of lead-induced stress corrosion cracking in the secondary side of steam generator tubes in nuclear power plants, comprising nickel boride, capable of inhibiting lead-induced stress corrosion cracking in the secondary side of steam generator tubes in nuclear power plants, and an inhibition method using the same.
However, the above conventional techniques do not mention a method of inhibiting lead-induced stress corrosion cracking in the secondary side of steam generator tubes in nuclear power plants, comprising supplying nickel boride as an inhibitor of stress corrosion cracking of steam generator tubes to a secondary side cooling water system in nuclear power plants.